The present invention is concerned with a novel process for the preparation of thiazolidinedione derivatives, especially with the preparation of 5-{4-[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophen-7-ylmethyl}2,4-thiazolidinedione and its salts. 5-{4-[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophen-7-ylmethyl}2,4-thiazolidinedione and its salts, particularly the sodium salt, are pharmaceutically active compounds. These compounds are known in the art and are described for example in International Patent Application WO 94/27995. They are especially useful for the prophylaxis and/or treatment of diabetes mellitus type I and II.
Methods for the preparation of 5-{4-[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophen-7-ylmethyl}2,4-thiazolidinedione have e.g. been described in WO 94/27995, WO 01/79202 and EP 1078923. However, these methods include a large number of individual reaction steps. Further, the methods known in the art exhibit a low yield or other disadvantages, which makes them unsuitable for the commercial large scale production of 5-{4-[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophen-7-ylmethyl}2,4-thiazolidinedione. A need exists for new methods of producing 5-{4-[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophen-7-ylmethyl}2,4-thiazolidinedione

that are economical and result in high yield of product.